Each day, the Earth receives on the order of 15 billion terajoules (TJ) of solar radiation. Approximately 30% of this is immediately reflected, refracted or scattered back into space, while the planet’s surface or atmosphere absorbs the remainder before re-emitting it.

Viewed from space and based on the absorption rate, the theoretical average temperature on the Earth should be -18°C, not the 14°C that was the 1951 to 1980 annual global mean.

What keeps the bulk of the planet’s surface above freezing is the greenhouse effect. The vast majority of gases in the Earth’s atmosphere are transparent to electromagnetic radiation at the frequency distributions emitted by the Sun and the Earth. But other, trace gases are responsible for the retention of heat: these greenhouse gases (GHGs) are essential for life as we know it.

CO2 and other long-life GHGsThe IEA publication CO2 Emissions from Fuel Combustion (2012) calculated global CO2 emissions from fossil fuel combustion to be more than 30.5 gigatonnes (Gt) in 2010, equivalent to atmospheric concentrations of 3.9 parts per million (ppm). The United States Department of Energy’s Carbon Dioxide Information Analysis Center has data extrapolated back to 1750 showing that of the 1 337 Gt of CO2 emitted from use of fossil fuels in the 261 years since the dawn of the steam age, 24% occurred in the 11 years from 2000 through 2010.

Oceans and other carbon sinks absorb some of that extra CO2, decreasing the ocean’s natural alkalinity in the process. But most of it remains in the atmosphere, increasing CO2 concentrations from 280 ppm in 1750 to 391 ppm in 2011.

The efficacy and atmospheric longevity of all these gases can be calculated in terms of an equivalent amount of CO2 (generally based on a 100-year period). Some have a global warming potential thousands of times more per molecule than CO2 at current concentrations, but they are not as large a by-product of human activities.

The 391 ppm CO2 figure comes from the US National Oceanic and Atmospheric Administration’s annual greenhouse gas index. But, using some estimates, all Kyoto GHGs totalled 446 ±6 ppm CO2-equivalent (CO2-eq). All long-life GHGs, which include chlorofluorocarbons, were 474 ±8 ppm CO2-eq.

What other GHGs are there?Ozone is not a long-life GHG, but it is a GHG. A naturally occurring gas in the stratosphere, it absorbs electromagnetic radiation (EMR) emitted by the Earth very effectively in a window of frequencies where very few other gases do. Human-caused emissions of ozone-depleting gases have decreased ozone concentrations in the stratosphere, creating a small cooling effect, or a negative radiative forcing (RF; see below). But a significant increase in tropospheric ozone production (caused by the release of other pollutants) ensures a solid overall warming effect even though ozone survives only a brief three weeks in the troposphere. The human effect on ozone concentrations currently results in an RF of about 0.31 Watts/m2 (W/m2), equivalent to approximately 23 ppm CO2-eq.

Water vapour is the most prevalent GHG, and the main contributor to the naturally occurring greenhouse effect. With concentrations tending to range between 1% and 4% of the atmosphere at sea level, water constitutes about 0.4%, or 4 000 ppm, of the atmosphere as a whole. It, too, is not a long-lasting GHG, as the hydrological cycle – via clouds and precipitation – ensures that water vapour lasts for just days in the troposphere. Anthropogenic hydrological warming effects include higher tropospheric concentrations of water vapour resulting from the effects of irrigation (which are still being quantified). In the stratosphere, concentrations have been changing of late for several reasons, including the decomposition of methane, increasing the greenhouse effect.

The mixed impact of aerosolsMade up of many components, aerosols have both the largest negative impact on RF and the largest uncertainty range. Like water vapour and ozone, they are naturally occurring phenomena in our environment, but human activities alter the type, pervasiveness and persistence of aerosols’ concentrations. Their effects can be classified as direct (i.e. the scattering and absorption of radiation) and indirect (providing nucleation that forms more clouds or clouds with different properties, increasing their albedo, or reflectivity). Currently, the anthropogenic indirect effect from aerosol particles contributes about 1% to the overall cloud albedo effect.

Direct effects from other sources (such as mineral dust from mining) are also often minor fractions of natural equivalents. Exceptions stem from combustion of fossil fuels: black carbon particles provide a notable heating influence when airborne, and anthropogenic sulphate emissions are several times larger than the amount of airborne dimethyl sulphide produced by phytoplankton.

The global pictureThe combination of all these variables and many more result in a change in RF from 1750 to 2011 of 2.07 W/m2, or 409.4 ppm CO2-eq, up from 406.4 ppm CO2-eq in 2010. The other factors include anthropogenic forcings like the effects of deforestation and small natural changes in how much radiation the Earth receives from the Sun. But not all anthropogenic effects are currently counted, as they are still being quantified; exclusions include effects from aviation.

There is a considerable delay between changes in RF and the climate system’s absorbing sufficient heat to respond fully. Studies show that the oceans have taken up more than 90% of the extra heat absorbed to date – well over 200 billion TJ from 1955 through 2003. By contrast, the total heat generated from combusting fossil fuels (not only useful heat but waste, too) was 11.5 billion TJ over those 49 years.

James Hansen was the lead author of a 2005 study that used data through 2003 to quantify the Earth’s thermal imbalance at 0.85 ±0.15 W/m2. That means that the planet was absorbing somewhere between 360 TJ and 510 TJ more every second around 2003 than it would normally re-emit. The implication is that the Earth’s climate must heat up by 0.6°C to address this imbalance, based on a climate sensitivity assumption of 0.75°C per W/m2. But that temperature change will take decades – during which time the RF imbalance most likely will increase further.

The ramifications of this imbalanceAverage annual GHG emissions have increased substantially since 2003, with atmospheric CO2 concentrations alone rising by more than 2 ppm per year at present. Adding 1 ppm of CO2-eq to the atmosphere effectively contributes about 7 TW of heating – approximately twice the capacity of the world’s collective fossil-fuel electricity generation systems.

Globally, according to data collected by the Goddard Institute of Space Studies in the United States, the 20 hottest years since 1880 have all occurred in the 24 years dating back to 1988, and the hottest decade on record (2001-10) was 0.23°C warmer than the second-hottest decade (1991-2000) and 0.55°C hotter than the 1951-80 benchmark.

Other feedback mechanisms including terrain changes such as the thawing of permafrost and albedo changes from melting ice will exacerbate the warming of the Earth and, in turn, increase the concentration of GHGs, setting up further warming.

Some measures to reduce emissions can have unwanted side effects. In the early 1970s, concerns over aerosol-induced global dimming, acid rain and other health and environmental problems led countries to use engineering measures to reduce particulate emissions. Germany, for example, cut its sulphur pollution by more than 90% in 30 years. But according to the Intergovernmental Panel on Climate Change’s Fourth Assessment Report, anthropogenic aerosols could still be providing -1.2 W/m2of forcing – that is, a cooling effect – about three-quarters of which can be attributed to sulphates. Given the short lifespan of tropospheric aerosols (hours to weeks), halving this concentration, while necessary, would be the equivalent of adding an extra 50 ppm CO2-eq to the atmosphere in a short time frame.

Some sobering implicationsDuring the last great climate-change event, the Paleocene-Eocene Thermal Maximum, some 55 million years ago, the Earth heated up about 6°C over 20 000 years and then took 200 000 years to return to near pre-perturbed conditions. The planet underwent significant changes during this period; ocean acidification accompanied mass extinctions on land and in the sea. The sedimentary rocks that formed before and after this event are so different that they were divided into separate geological epochs long before scientists discovered what had caused the split.

For a great many species, adaptation was simply not possible, even over many thousands of generations. Business-as-usual scenarios using current emission trajectories suggest that a similar degree of climate change is likely, but over less than 1/100th the amount of time.

What is radiative forcing?

EMR intensity depends upon the temperature of the object emitting it. EMR absorbed by the Earth and the total EMR that the Earth emits must be equivalent for the planet to neither gain nor lose energy. Altered factors (such as an increase in greenhouse gas concentrations) that change this radiative balance are known as “forcings”, as they demand a reaction. They are usually calculated showing their effect at the tropopause (see map of atmosphere on last page of article), and are measured from a start time (e.g. 1750). As potential flows of energy, they tend to be expressed in Watts per square metre

Julian Smith, who studied science at the University of Queensland in Australia, joined the IEA Energy Data Centre in 2008, focusing on electricity, solid fossil fuels and carbon capture and storage. He is a collaborator on the IEA Coal Information book and assists the CCS Technology unit.

The International Energy Agency (IEA) produces IEA Energy, but all analysis and views contained in the journal are those of individual authors and not necessarily those of the IEA Secretariat or IEA member countries, and are not to be construed as advice on any specific issue or situation.